This invention relates to an improved lifting device for down hole tubulars for use in combination with an earth drilling machine of a type having a boom for moving a down hole tubular from a horizontal position at an intermediate level to a raised position aligned with the drilling axis.
Drilling machines of the type described above are known to the art, as disclosed for example in U.S. Pat. Nos. 4,708,581 and 4,407,629. In certain applications it is preferable to pivot the boom about an axis which is elevated a substantial distance above ground level, and above the level at which down hole tubulars are stored near ground level. In the past, lifting devices have been provided for lifting individual lengths of down hole tubular from a lower level (characteristic for example of a pipe rack) and the intermediate level (where the down hole tubular may be loaded into the boom). One prior art system utilizes two hydraulic cylinders, each positioned to lift the tubular at respective positions along the length of the tubular. A flow divider divides the flow of hydraulic fluid between the two cylinders.
One drawback of this prior art approach is that in the event a tubular bears much more heavily on one of the two cylinders, that cylinder may be slower to raise its end of the tubular. If the two cylinders get out of synchronization to any substantial extent, there is a danger that the tubular may slip along its length, thereby mispositioning the tubular and complicating further tubular handling operations.
It is an object of this invention to provide an improved lifting device which overcomes these problems of the prior art.